Alfa's Hemispherical chambers and the Dodge Hemi - Page 4

ddouglas77 · #46 (**permalink**) 10-21-2006, 10:33 PM

Hemi stands for hemispherically charged combustion chamber, basically, most cars today are hemi's, when the valves surround the spark plug-its a hemi. Look at any DOHC and that is exactly what you see.

BigSwede · #47 (**permalink**) 10-22-2006, 04:15 AM

Not really, all 4 valve heads are basically Pent-roofs. A 4v hemi would be incredibly complicated and add nothing.

gtv2000 · #48 (**permalink**) 10-22-2006, 09:58 AM

Update on the possible first application of a hemi head:

I found that, indeed, Brussel's company Pipe presented in 1904 an interesting bicylindre car with OHVs in a hemi configuration. But the same year, an engineer called Welch (company is named Welch or Chelsea, seemingly, and was based in Pontiac, USA) also built a car with similar features. An earlier application would date back to 1898, credited to an engineer Wilkinsons, abouth whom I found nothing up to now. Even the DOHC first automobile application should date around 1905, before its first really efficient use by Henry on the GP Peugeot 1912.

The story is not yet clear nor complete, but in any case we are a good 50 years before the "revolutionary hemi head".

dretceterini · #49 (**permalink**) 10-22-2006, 10:52 AM

Just a thought.

If modern racing cars are using dished pistons, where the combustion chamber is really the top of the piston, why hasn't there been more experimentation into this design for street cars?

ddouglas77 · #50 (**permalink**) 10-22-2006, 12:16 PM

Quote:

Originally Posted by BigSwede

Not really, all 4 valve heads are basically Pent-roofs. A 4v hemi would be incredibly complicated and add nothing.

BigSwede · #51 (**permalink**) 10-22-2006, 01:54 PM

Quote:

Originally Posted by ddouglas77

I did not say anything about a 4 valve head-I said dohc, like the alfa, even in 4 valve heads, as long the spark occurs from the upper most centered part of the head-it is a hemi.

ddouglas77 · #52 (**permalink**) 10-22-2006, 03:29 PM

Yeah, well thats why I'm an engineer, to make life for ignorance harder....Pent roof-ha.

Greg Gordon · #53 (**permalink**) 10-22-2006, 04:03 PM

Stu, I am not sure I understand what you mean. However the old Chevrolet 348 and 409 series had overhead valves with a flat surface where the valves were and the shape of the combustion chamber was built into the piston. That's the only type of engine I have seen like that. Is that's what you are talking about. If so what modern engine is set up like that?

**genericwood** · #54 (**permalink**) 10-22-2006, 04:14 PM

Mats, how dare you tell him he is wrong? Even he though he clearly is wrong, you don't have to tell him so!

Erik

Quote:

Originally Posted by ddouglas77

Yeah, well thats why I'm an engineer, to make life for ignorance harder....Pent roof-ha.

ddouglas77 · #55 (**permalink**) 10-22-2006, 04:18 PM

This is beyond ridiculious-give me something real-a hemi, is a hemi, by design, the spark plug has to be centrally located within the combustion chamber of the head-as the "old school" photo shows, a lot of people here talk like they know what they're saying-adding technical jargon that has no real basis-if you don't know what you're saying-then shut up! I got your "pent roof" right here buddy!

Greg Gordon · #56 (**permalink**) 10-22-2006, 04:38 PM

Mats is one of the most knowledgeable people here. I think you guys are getting caught up in semantics. While a hemi head motor does have its spark plug centrally located that's not the only factor that makes it a hemi. If it was then all the Buick 401s and 425s as well as the Fiat 124's dohc motors would be hemis. They are not they are pentroofs.

ddouglas77 · #57 (**permalink**) 10-22-2006, 05:04 PM

I can't argue with that, the basic principles as outlined in the image I have provided earlier-shows clearly what a hemi (if you want to talk about Dodge Hemis and how these engines are not property of Dodge-so be it-they're not). When talking about a hemi-the #1 rule is not only the location of the plug but the valves to it. I am not knowledgeable on Alfas, but I am an automotive engineer for GM-so don't try to tell me that I'm wrong when applying that to the basic rough draft of a combustion chamber-it don't fly-even if you add wings.

**papajam** · #58 (**permalink**) 10-22-2006, 05:10 PM

Quote:

Originally Posted by ddouglas77

...by design, the spark plug has to be centrally located within the combustion chamber of the head...

Greg Gordon · #59 (**permalink**) 10-22-2006, 05:13 PM

Image 1 shows the valve positions in a hemi engine, not combustion chamber shape. In other words that picture could be a hemi or a pentroof as the valve positions can be pretty much the same in a 2 dimensional side view.

Pentroofs and Hemis are very similar but are not quite the same. In fact during WW2 the N.A.C.A. did very detailed studies on the detonation characteristics of the various types of combustion chambers. In these reports they clearly considered these to be two different combustion chamber designs.

ddouglas77 · #60 (**permalink**) 10-22-2006, 05:14 PM

Once again-centered when compared to the placement to the valves-now since there are two plugs-you obviously can't have both in dead center. The pic I had placed earlier shows the relationship between valve and plug-clearly-if yours in one way or another relate to pic B-you have a hemi-other than that-no, I'm not familiar with Alfas-I'm here because I'm trying to be, but overall design is the same...I could say a dual piston caliper has 2 pistons, as soon as I say that-someone here will say that they don't and start ranting and raving naming things that do not exist.